1. Technical Field
The present invention relates to a tripod type constant velocity universal joint and a method of manufacturing the tripod type constant velocity universal joint.
2. Background Art
In a constant velocity universal joint, which is used to construct a power transmission system for automobiles and various industrial machines, two shafts on a driving side and a driven side are coupled to each other to allow torque transmission therebetween, and rotational torque is transmitted at a constant velocity even when each of the two shafts forms an operating angle. The constant velocity universal joint is roughly classified into a fixed type constant velocity universal joint that allows only angular displacement, and a plunging type constant velocity universal joint that allows both the angular displacement and axial displacement. In a drive shaft for transmitting power from an engine of an automobile to a driving wheel, for example, the plunging type constant velocity universal joint is used on a differential side (inboard side), and the fixed type constant velocity universal joint is used on a driving wheel side (outboard side).
As an example of the plunging type constant velocity universal joint, a tripod type constant velocity universal joint has been known. As this tripod type constant velocity universal joint, which includes rollers serving as torque transmitting members, a single roller type and a double roller type have been known. FIGS. 12 to 15 exemplify a tripod type constant velocity universal joint of the double roller type (refer, for example, to Patent Document 1).
FIG. 12 is a partial vertical sectional view of the tripod type constant velocity universal joint, and FIG. 13 is a partial horizontal sectional view taken along the arrow K-K in FIG. 12. As illustrated in FIGS. 12 and 13, a tripod type constant velocity universal joint 101 includes, as a main part, an outer joint member 102, a tripod member 103 serving as an inner joint member, and roller units 104 serving as torque transmitting members. The outer joint member 102 is formed into a cup shape that is opened at one end, and has an inner peripheral surface having three linear track grooves 105 equiangularly formed therein so as to extend in an axial direction. On both sides of each of the track grooves 105, there are formed roller-guide surfaces 106 arranged to face each other in a circumferential direction and each extending in the axial direction. The outer joint member 102 receives therein the tripod member 103 and the roller units 104. The tripod member 103 includes three leg shafts 107 projecting in a radial direction. A shaft 109 is spline-fitted to a center hole 108 of the tripod member 103, and fixed in the axial direction with a stopper ring 110. The roller units 104 each include, as a main part, an outer ring 111 serving as a roller, an inner ring 112 arranged inside the outer ring 111 and externally fitted to the leg shaft 107, and needle rollers 113 interposed between the outer ring 111 and the inner ring 112. The roller units 104 are received in the track grooves 105 of the outer joint member 102. The inner ring 112 has an inner peripheral surface 112a formed into a convex circular-arc shape in vertical cross-section including an axial line of the inner ring 112. The roller unit 104 has a structure in which the inner ring 112, the needle rollers 113, and the outer ring 111 are prevented from being separated from each other with washers 114 and 115.
The leg shafts 107 of the tripod member 103 each have an outer peripheral surface formed into a straight shape in vertical cross-section including an axial line of the leg shaft 107. Further, as illustrated in FIG. 14 corresponding to a plan view as viewed from the arrow L-L in FIG. 12, the outer peripheral surface of the leg shaft 107 is formed into a substantially elliptical shape in horizontal cross-section that is orthogonal to the axial line of the leg shaft 107, and is held in contact with the inner peripheral surface 112a of the inner ring 112 in a direction that is orthogonal to an axial line of the joint, that is, a direction of a major axis “a” in a manner that clearances m are formed between the outer peripheral surface of the leg shaft 107 and the inner peripheral surface 112a of the inner ring 112 in a direction of the axial line of the joint, that is, a direction of a minor axis “b”. In the constant velocity universal joint 101, the outer ring 111 of the roller unit 104 mounted to the leg shaft 107 of the tripod member 103 rolls on the roller-guide surfaces 106 of each of the track grooves 105 of the outer joint member 102. The leg shaft 107 is formed into a substantially elliptical shape in horizontal cross-section. Thus, as illustrated in FIG. 15, when the constant velocity universal joint 101 forms an operating angle, an axial line of the tripod member 103 is inclined with respect to an axial line of the outer joint member 102. Meanwhile, the roller unit 104 can be inclined with respect to the axial line of the leg shaft 107 of the tripod member 103, and hence the outer ring 111 of the roller unit 104 and the roller-guide surfaces 106 can be avoided from obliquely crossing each other. With this, the roller unit 104 properly rolls. Thus, inductive thrust and sliding resistance can be reduced, with the result that vibration of the joint can be reduced.
Next, description is made of a manufacturing step for the tripod member 103. In the tripod member 103, as illustrated in FIGS. 18a and 18b, a forged product 103′ of the tripod member 103 is formed through full-enclosed forging using a die set formed of an upper die 120, a lower die 121, an upper punch 122, and a lower punch 123. Specifically, the upper die 120 and the lower die 121 are clamped to each other so as to define a forming space, and a cylindrical billet is put therein. Then, the upper punch 122 and the lower punch 123 are brought close to each other so that the billet is pressurized and filled between the dies 120 and 121. In this way, the forged product 103′ including three leg shafts 107′ is obtained. After that, the forged product 103′ is finished through a machining process to have a spline hole, end portions of the leg shafts, and the like, and then is subjected to heat treatment.
After the heat treatment step, as illustrated in FIGS. 16 and 17, outer peripheral surface regions of the leg shaft 107, which are to be held in contact with the inner peripheral surface 112a of the inner ring 112 (refer to FIG. 14), are finished through grinding. FIG. 16 illustrates the leg shaft 107′ under a state in which the heat treatment is completed. As illustrated in FIG. 16, the outer peripheral surface of each of the leg shafts 107′ includes outer peripheral surface regions 107a′ including the major axis “a” of the substantially elliptical shape. A permissible dimension of a grinding width of each outer peripheral surface region 107a′ is denoted by a reference symbol A. A range of slow grinding feed along with rotation about an axial center O of the leg shaft 107′ is denoted by a reference symbol B. Although not shown, in a process of this grinding, the forged product 103′ is chucked with a grinding apparatus, and the leg shaft 107′ is rotated, for example, in a counterclockwise direction about the axial center O of the leg shaft 107′ so that the outer peripheral surface regions 107a′ including the major axis “a” of the elliptical shape are ground with a grinding stone. At this time, in order to form the outer peripheral surface regions 107a′ into the elliptical shape, the grinding stone is slightly advanced and retreated in synchronization with the rotation of the leg shaft 107′ about the axial center O so that the outer peripheral surface regions 107a′ are subjected to the grinding process into the elliptical shape.
A required grinding range corresponds to the permissible dimension A in FIG. 16. However, in consideration of inevitable variation in forging dimension and heat treatment deformation, the grinding feed range B is set to be significantly wider than the permissible dimension A of the grinding width. However, detailed description thereof is given later. A speed of the rotation of the leg shaft 107′ about the axial center O at the time of the grinding process is set so that the slow grinding feed is performed in a range of an angle C corresponding to the range B, and that fast feed is performed without grinding in a range of an angle D. FIG. 17 illustrate the tripod member 103 as a finished product obtained by finishing outer peripheral surface regions 107a of the leg shafts 107 through grinding in this way. FIG. 17a is a partial vertical sectional view of the tripod member 103, and FIG. 17b is a right side view of FIG. 17a. Parts that are finished through grinding as illustrated in FIG. 17 correspond to the outer peripheral surface regions 107a including the major axis “a” of the substantially elliptical shape. When a grinding width that is equal to or larger than the permissible dimension A is secured, the tripod member 103 is regarded as a non-defective product.